I have tested with the RelayDemo as the Description in the jgroups manual together with the udp.xml from the 3.6.9 jgroups Version.
At the Moment if the bridge-cluster is aktiv, the system sent continuously null messages to all Members.
if we replace pbcast.NAKACK2 with pbcast.NAKACK the demo is ok.
So i have seen it is planned to remove NAKACK in the Future - i create this issue.
Thanks for the nice software!